In recent rotational recording apparatuses employing a magnetic disk, etc., data-transfer velocity is rising upon increase of their storage capacity. Hence, since a disk rotating apparatus employed in such recording apparatuses requires high-speed and high precision rotations, a hydrodynamic bearing device is used in a rotary main shaft portion of the disk rotating apparatus.
Hereinafter, a conventional hydrodynamic bearing device is described with reference to FIGS. 18 and 19. In FIG. 18, a shaft 31 is rotatably inserted into a bearing bore 32A of a sleeve 32 mounted on a base 35. In FIG. 18, the shaft 31 has a flange 33 formed integrally at its lower end portion. The flange is received in a step portion 32K of the sleeve 32 so as to rotatably confront a thrust plate 34. A rotor hub 36 to which a rotor magnet 38 is secured to is attached to the shaft 31. A plurality of disk 39 held by a spacer 40 and a damper 41 are mounted on the rotor hub 36. A motor stator 37 confronting the rotor magnet 38 is mounted on the base 35. Dynamic pressure generating grooves 32B and 32C are provided on an inner peripheral surface of the bearing bore 32A of the sleeve 32. Dynamic pressure generating grooves 33A are provided on one face of the flange 33, which confronts the step portion 32K of the sleeve 32, while dynamic pressure generating grooves 33B are provided on the other face of the flange 33, which confronts the thrust plate 34. Clearances between the shaft 31 and the flange 33 on one hand and the sleeve 32 on the other hand, which include the dynamic pressure generating grooves 32B, 32C, 33A and 33B, are filled with oil 42. One or more vent holes 32E are provided on the sleeve 32 substantially in parallel with an axis of the sleeve 32. A lower end of the vent holes 32E communicates with a space which is disposed at a lower end portion of the sleeve 32 so as to contain the flange 33. An upper end of the vent holes 32E opens to an upper end face of the sleeve 32.
Operation of the conventional hydrodynamic bearing device of the above described arrangement is described by referring to FIGS. 18 and 19. In FIG. 18, if the motor stator 37 is energized, a rotary magnetic field is generated and thus, the rotor magnet 38, the rotor hub 36, the shaft 31 and the flange 33 start rotations. At this time, a pumping pressure is generated in the oil 42 by the dynamic pressure generating grooves 32B, 32C, 33A and 33B. Thus, the shaft 31 is raised and is rotated without coming into contact with the thrust plate 34 and the inner peripheral surface of the bearing bore 32A while being lubricated by the oil 42. A magnetic head (not shown) is brought into contact with the disks 39 so as to perform recording and reproduction of electrical signals.
The above conventional hydrodynamic bearing device has the following problems. FIG. 19 is a fragmentary sectional view including the shaft 31 and the sleeve 32 of FIG. 18. As shown in FIG. 19, the shaft 31 is rotated in the bearing bore 32A of the sleeve 32 while being lubricated by the oil 42. When the hydrodynamic bearing device has been assembled or while the hydrodynamic bearing device is being transported, air lumps or air bubbles (hereinafter, referred to as “air 43A or 43B”) may penetrate into the oil 42 in the bearing bore 32A. For example, in case ambient pressure has changed during transport in an aircraft, penetration of air bubbles may happen. If volume of the air 43A penetrating into the vicinity of the dynamic pressure generating grooves 32B and 32c is expanded by rise of temperature or drop of atmospheric pressure, a portion of the dynamic pressure generating grooves 32b is covered by air, thereby resulting in absence of the oil film. Meanwhile, a portion of the oil may leak out of the hydrodynamic bearing device as indicated by oil 42B. Meanwhile, if the air 43B penetrating into the vicinity of the flange 33 is expanded, the hatched oil 42A in the vent hole 32E may be pushed upwardly by expanded air 43C so as to leak out of the hydrodynamic bearing device as shown by oil 42D. If the oil 42 leaks outwardly, shortage of quantity of the oil in the bearing occurs. As a result, there is a risk of extreme aggravation of reliability due to contact of the shaft 31 with the sleeve 32 during rotation.
Meanwhile, also in case a drop impact load (acceleration) is applied to the conventional hydrodynamic bearing device in the direction of the arrow G1 as shown in FIG. 19, there is a risk that the oil 42 leaks outwardly as shown by the oil 42B.